Two seats and turboprop performance
It is, in certain respects, the ultimate kit-built spar. The leading edge is stretch-formed,
airplane. The fastest, with a top speed of 239 meaning that it is stretched into shape by
knots. The most expensive, at $43,021 for a pressing a mold into a sheet of aluminum.
series of six construction kits (not including This technique enables the leading edge con-
engine, propeller, avionics, instrumentation, tours to be formed with a high degree of con-
upholstery and paint). And perhaps the most formity to design specifications.
complex to build. For many pilots, the Rear wing skins join the leading edge in a
Swearingen SX300 may represent the ulti- joint that is recessed one tenth of an inch
mate personal airplane available from any below the aerodynamic contour of the wing.
source, whether it be a factory production This depression is filled in with a synthetic
line or a kit-maker's packing crate. compound that is flexible when dried and
The SX300 is the invention of designer/ will not crack, according to Swearingen.
entrepeneur Edward J. Swearingen Jr., who is Swearingen, the designer alld the design Carefully applied and sanded before paint-
best known for the line of aircraft that bears ing, the filler is almost unnoticeable, even un-
his name-the Swearingen Merlin and Metro knots, slowing to 80 over the threshold. Stall der close inspection, and the wing takes on a
turboprops. Swearingen sold the line to the speed in the landing configuration is 70 knots seamless look.
Fairchild Aircraft Corporation 15 years ago, at gross weight. With gear and flaps up, stall The wing section is a natural laminar flow
and updated versions of those aircraft now speed rises to 75 knots. (NLF) design, an NLF(1)-0416, to be precise.
are marketed as the Fairchild 300, 400 and It might seem that combining a big engine As the airfoil's designer, NASA engineer Dan
Metro. Among other projects, Swearingen with a small airframe would be a simple for- M. Somers, explains it, NLF airfoils, designed
also was involved in the development of the mula for high performance. The trick, of with the aid of computers, offer the low drag
Twin Comanche, the Cheyenne and a proto- course, is to make the airplane an efficient, factors of earlier laminar-flow designs com-
type for a pre-Malibu pressurized single sturdy and stable machine. The engineering bined with the high lift coefficients of turbu-
while working under contract to Piper Air- that has gone into the SX300 is more typical lent-flow airfoils. Earlier laminar-flow de-
craft in the 1960s and early 1970s. of the approach taken in designing a business signs could not match the high lift produced
For Swearingen, speed always has been a jet than a general aviation single. by non-laminar-flow airfoils. Because of the
high design priority. The keys to superior Wing and fuselage skins are heavy-gauge high-lift properties of the SX300's wing sec-
performance, in his view, are plenty of .040-inch-thick aluminum alloy. The skins tion, the wing is set at a slightly negative
power, high wing loading, a conventional account for much of the airframe's strength, angle of incidence (minus one degree at the
configuration and meticulous attention to reducing the need for complex and heavy in- root, minus four degrees at the tip) in order to
drag reduction. That philosophy, and a dedi- ternal structures. The wing has been static- produce the least drag in cruise flight.
cation to the highest standards of construc- load tested to six Gs. Stress analysis calcula- Rush rivets are used throughout the air-
tion, are reflected in every inch of the SX300. tions indicate that the failure point is beyond frame. Communications and navigation an-
The 300-horsepower Lycoming 10-540 nine Gs. Wing leading edges are fabricated as tennas are buried in the fiberglass wing tips
that powers the SX300 is one of the largest single U-shaped sections, spanning from and in the tail. The gear is operated by means
horizontally opposed piston engines found wing root to wing tip and curving aft to the of an electrically actuated hydraulic system
on general aviation singles; it is bolted to one
of the smallest airframes. Wingspan is 10 feet
less than a Piper Tomahawk's, and length is
one foot shorter. The SX300's power to
weight ratio of eight pounds per horsepower
places the airplane in a class with some turbo-
props, and so does its performance.
Best initial rate of climb is 2,400 feet per
minute at 125 knots. A cruise climb speed of
160 knots yields about 1,000 fpm below
10,000 feet. Cruise speed at 65-percent power
and 12,500 feet is 231 knots, and range is 980
nautical miles with a 45-minute reserve.
Maneuvering speed in the SX300 is a high
207 KIAS, which allows the pilot to descend
without having to make substantial speed re-
ductions. It also allows the pilot to maintain a
relatively high speed in turbulence. The day I
flew Swearingen's prototype, the tiny air-
plane was hardly budged by strong midday
Texas thermals, a consequence of its high
33.6-pounds-per-square-foot wing loading.
Trade-offs for the SX300's impressive top-
end performance figures come in the form of
high (for a single) approach and stall speeds.
Recommended final approach speed is 100
62· AUGUST 1986
and is fully enclosed when retracted. The through the cylinder heads more efficiently. The airplane's aerobatic capability will
flaps are hydraulically actuated. Several other modifications have been tempt pilots. But a high degree of aerobatic
Minimizing cooling drag while providing made to the SX300 since initial flight testing. proficiency will be required to avoid exces-
even cooling to all cylinders was a significant The horizontal tail area was increased ap- sive speed build-up or inadvertent spins. At
design objective. At the SX300's cruise proximately 20 percent to improve low-speed the moment, the airplane's spin characteris-
speeds, intake air can actually "dam up" at pitch control. Rudder chord was increased tics are unknown. Swearingen says spin tests
the cooling air inlets, adding substantial drag. two inches to provide more control in a full- will be conducted in the future, after the air-
The solution was to modify the prototype's power stall. Positive servo tabs were added to plane has been rigged with a spin recovery
original cowling by extending the cooling air the ailerons to reduce roll forces, and an anti- parachute and a canopy jettison system.
inlets forward almost to the prop. The origi- servo tab was added to the elevator to in- The stall break may seem rather sharp to
nal nearly rectangular inlet shape was re- crease pitch forces. Wing dihedral was in- pilots accustomed to the stall characteristics
jected in favor of a rounded inlet, and the creased from three to four degrees to improve of factory-built high-performance singles.
inlet leading edges were molded in an airfoil roll stability. Overzealous application of the SX300's sensi-
shape. Felt seals surround the propeller shaft Inside the cockpit, the prototype's soft tive controls could lead the pilot to exacerbate
to prevent cooling air from reversing course leather upholstery adds an appropriate touch the stall. Again, pilot proficiency will be the
and flowing back out the front of the cowl. of luxury to such an exotic airplane. But the key to safe stall recovery.
The upper half of the powerplant is tightly important inside story has to do with Swearingen Aircraft has shipped three of
baffled in order to direct cooling air down Swearingen's attempt to provide substantial the aircraft's six subassembly kits to build-
system redundancy. The pilot's gyro instru- ers-those for the empennage, wings and fu-
ments are electrically driven, while the co- selage. Shipments of the engine installation
pilot's gyros are air-driven. Neither an electri- and landing gear kits are scheduled to com-
cal failure nor a vacuum failure will leave the mence in September 1986. The cockpit com-
pilot without attitude and direction reference. pletion kit will be the last to be shipped. Kits
A split-bus electrical system allows the pi- have been shipped to 88 individuals, 69 of
lot to quickly shed electrical load while re- whom are actively building airplanes, accord-
taining essential items-the pilot's turn co- ing to Swearingen.
ordinator, one navcom, the fuel boost pump SX300 builders interviewed for this article
lllllovatioll alld attelltioll to detail are and a map light. The prototype also is praise the quality of materials and prefab-
reflected ill the distillctive shape of a equipped with a two-axis S-Tee autopilot ricated parts. Drawings and assembly instruc-
Ilaturallami/Jar flow airfoil alld the 5X300's specially programmed for the pitch and roll tions for the airplane are first-rate, and tech-
roullded, airfoil-shaped air illiets. characteristics of the SX300. nical support from Swearingen Aircraft has
Flying characteristics are sophisticated, been good, they say. But some also say that
also. Prospective pilots need training in truly the SX300 is as complex and demanding a
high-performance aircraft; transition training building project as one is likely to find among
tailored to the SX300 would be even better. It kit aircraft.
is not an aircraft that should be treated lightly Swearingen will not estimate building time
in allY aspect of its operation. for the SX300 because, he says, it will vary
Even with the addition of the anti-servo considerably with the experience of the
tab, pitch forces are quite light in all phases of builder. One spare-time builder, who expects
flight. Aileron and rudder forces are as nicely to take at least five years to finish his airplane,
balanced as they are responsive. Roll rate is quips that "The SX300 is for the buyer who
140 degrees per second, and aileron rolls are wants a fast airplane, but doesn't need an
nearly effortless. airplane fast." Some purchasers have hired
experienced aircraft mechanics or home-
builders to work full-time on the construction
of their SX300s.
Building time may be reduced substan-
tially, however, if Swearingen wins FAA ap-
proval for his plan to offer what he refers to as
the SX300 "big chunk" kit. This kit would
provide a fully assembled fuselage, com-
pleted wings, and an empennage complete
except for the left-hand stabilizer-the only
component that will have to be fashioned out
of sheet metal. Rather than spending hun-
dreds of hours drilling holes and driving riv-
ets, the builder will bolt the airframe together
and install the various aircraft systems.
The kit still will meet the amateur-built re-
quirements, Swearingen says, because the
homebuilder still puts in more than half the
hours of labor necessary to complete the air-
plane, while learning just as much about air-
craft construction as with the previous kit.
(FAA regulations require that amateur-built
airplanes be assembled solely for education
or recreation.) A price has not yet been ar-
rived at for the new kit, but it too will be a big
chunk, says Swearingen. -J. Jeffersoll Miller
AOPA PILOT • 63